1. Field of the Invention
This application claims the benefit of Chinese Patent Application No. 200710176603.7, filed Oct. 31, 2007, which is incorporated by reference herein.
The present invention relates to the field of test methods for eye refractometers using an objective measuring principle. More specifically, the present invention relates to a cylindrical model eye, a cylindrical test device and the manufacturing method thereof. The cylindrical test device is mainly used for testing the cylindrical vertex power and cylinder axis accuracy of eye refractometers.
2. Description of the Related Art
Dioptre inspection, also called optometry, means examination of an eye's refractive errors for a patient by a series of inspections. The device used for optometry is called an eye refractometer. The eye refractometer primarily measures the spherical vertex power (myopia or hyperopia), the cylindrical vertex power (astigmatism) and cylinder axis (i.e. axis of astigmatism) etc. of the eye.
Corresponding test devices have to be used for testing eye refractometers, to check data therefrom for accuracy and reliability. The test device for testing the spherical vertex power of an eye refractometer is called as a spherical model eye, and the test device for testing the cylindrical vertex power and cylinder axis of the eye refractometer is called as a cylindrical test device.
The spherical model eye, recommended in ISO 10342 “Eye refractometers”, is made of optical glass and is formed with a spherical front surface and a lightly frosted plano back surface. The spherical model eye is easy to be manufactured easily because that the processing technique for optical components with spherical surfaces is advanced.
As to the cylindrical test device, two solutions are recommended in ISO 10342 “Eye refractometers”. One solution is a model eye with a front toric surface. Another is addition of a thin rigid toric lens with an 8 mm base curve to the spherical model eye.
A first solution is a modification of the spherical model eye. It uses a toric surface instead of the spherical surface as the front surface.
The toric surface is a surface for which two main principal curvatures are unequal a lens with a toric surface will bring paraxial parallel light to two separate line foci mutually at right angles, hence having different vertex powers in two meridian planes. The cylindrical vertex power is defined as the difference between the two vertex powers in the two meridian planes.
In order to meet the requirements in ISO 10342 “Eye refractometers”, such a toric surface cylindrical model eye is made of optical glass. Furthermore, the curvature radii of the two main meridian planes are both nearly about 8 mm, and the manufacture thereof is difficult.
The second solution is advantageous because it is easy to manufacture the spherical model eye. However, it is difficult to manufacture a thin rigid toric lens with an 8 mm base curve. Experimentation with addition of well-known astigmatic contact lenses, to the spherical model eye has been done. The experiment shows the following disadvantages.
If a soft astigmatic contact lens is added to the spherical model eye, due to variable water content of the soft astigmatic contact lens, the shape and the dimensions thereof vary continuously, thus, the cylindrical vertex power and the cylinder axis of the combination of the soft astigmatic contact lens and the spherical model eye are unstable.
If a hard astigmatic contact lens is added to the spherical model eye, it is difficult to solve the coaxial and concentric problems after assembly due to the different design parameters and the limited manufacture capability of the hard astigmatic contact lens, and the testing results are influenced by the problems such as ghost image when the refractometer is detected.
Therefore, the practicability and reliability of the two conventional solutions is inadequate.